• We retrospectively reviewed the records of 12 patients from one institution (from 1970 through 1983) who had hyperinsulinism as a part of the multiple endocrine neoplasia, type 1 syndrome. All of the patients underwent surgical exploration: ten had subtotal pancreatectomies, one had enucleation, and one underwent a total pancreatectomy. There was no operative mortality. Preoperative and intraoperative localization studies (angiography and ultrasonography) tended to underestimate the number of pancreatic tumors. Ten patients had multiple B-cell adenomas (mean, 7.4), while two patients had single tumors. This study confirms that hyperinsulinism, when it occurs as part of the multiple endocrine neoplasia, type 1 syndrome, is usually due to multiple islet cell tumors, and that the islet cell tissue is probably diffusely dysplastic. Subtotal (85%) pancreatectomy is suggested as the procedure of choice for this subgroup of patients with hyperinsulinism.